1. Field of the Invention
The present invention relates to a process for the preparation of protective colloid-stabilized aqueous dispersions, and the use thereof.
2. Description of the Background
Dispersions comprising hydrophobic monomers, such as styrene or butadiene, which are stabilized with polar protective colloids, such as PVA or hydroxyethylcellulose (HEC), are difficult to prepare because they are very viscous (U.S. Pat. No. 4,670,505). These copolymers are as a rule therefore stabilized by more efficient ionic and nonionic emulsifiers which result in a lower viscosity in the process. The protective colloid imparts to the dispersion rheological and adhesion properties which differ from those imparted by the emulsifier, which also adversely affects the water resistance of the dispersion.
Protective colloid-stabilized dispersions can be dried by spray-drying to give a redispersible powder which results in good processibility and an improvement in the mechanical properties in hydraulically setting systems. Such redispersible powders are used in large amounts in construction applications. Virtually exclusively powders based on polar monomers, such as vinyl acetate or vinyl chloride, or vinyl acetate/ethylene copolymers or vinyl acetate/veova copolymers have been prepared to date. Owing to the higher polarity, the polymerization with polar protective colloids presents no problems and is not comparable with hydrophobic monomers, such as styrene or 1,3-butadiene.
Owing to the interesting properties, protective colloid-stabilized dispersions based on hydrophobic monomers and the possibility of spray-drying them to give a redispersible powder, a process for the preparation of protective colloid-stabilized dispersions based on vinylaromatics and 1,3-dienes is of considerable economic interest.
Freeing emulsifier-stabilized dispersions based on vinylaromatics and 1,3-dienes from residual monomers and readily volatile impurities by steam distillation is known. The residual monomer content at the end of polymerization is below 1%. In addition to a low process viscosity, the aim of the preparation process according to the invention for the protective colloid-stabilized dispersions based on hydrophobic monomers is to achieve a final conversion of  greater than 99%. This is of considerable economic interest since the larger the amount of residual monomers the greater are the resulting amounts of wastewater, which cause environmental pollution and give rise to disposal costs.
There are numerous known preparation processes for protective colloid-stabilized dispersions based on hydrophobic copolymers, such as styrene/acrylate or styrene/butadiene, which make it possible to control the viscosity in the process but which have disadvantages with respect to the process.
The viscosity of vinyl acetate/(meth)acrylate dispersions having a vinyl acetate content of  less than 50% with the use of PVA or HEC as protective colloid can be reduced by stabilizing auxiliaries, such as allyl alcohol (U.S. Pat. No. 4,670,505), propanol or ethylene glycol (British Patent 1278813). In EP-A 013 478, the viscosity is reduced with water-miscible organic compounds, such as methanol and ethanol. The volatile organic compounds have to be removed after the polymerization.
In EP-A 538 571, the total amount Of protective colloid is initially introduced for regulating the viscosity of dispersions having a styrene content of  greater than 50% by mass and/or C1- to C8-alkyl (meth)acrylate and a mixed initiator system consisting of an acidic peroxide and a redox system, or an acidic peroxide or a redox system alone is used. In addition, the viscosity can be reduced by using triethanolamine as a viscosity-regulating compound.
EP-A 821 016 and EP-A 723 975 refer to the viscosity-regulating effect of polar comonomers which are capable of crosslinking during the polymerization itself, such as hydroxyethyl methacrylate or glycidyl (meth)-acrylate, in the polymerization of styrene/acrylates with partially hydrolyzed polyvinyl acetate as protective colloid. The polymerization of styrene/butadiene copolymers is unsuccessful.
WO 99/16794 states that, by using a mixture of protective colloids which differ in the surface tension, it is possible to prepare sterically stabilized styrene/butadiene copolymer dispersions which can be spray-dried to give a cement-stable, redispersible powder. One protective colloid has a surface tension of  greater than 40 mN/m in the form of a 2% strength aqueous solution and the other has a surface tension of  less than 40 mN/m in the form of a 2% strength aqueous solution. The styrene/butadiene dispersion thus prepared has a solids content of only 47.0%, which corresponds to a conversion of 93.6%. According to the fundamental formulation in the example, the theoretical solids content would have to be 50.2% in the case of a conversion of 100%. In this process, the fact that a mixture of two different protective colloids has to be used is also disadvantageous.
The preparation of a sterically stabilized dispersion of a styrene/butadiene copolymer, which can be spray-dried to give a redispersible powder, using only one protective colloid is effected in WO 99/28360 by initially introducing some of the protective colloid and metering some. The very large particle sizes of the dispersion, from about 2.5 xcexcm to 4.5 xcexcm, constitute a disadvantage of this process. Experience has shown that dispersions having these particle sizes (substantially above 1 xcexcm) are not storage-stable. A part of the monomers has to be initially introduced and a part metered. If all monomers are added during the polymerization, the conversion of the polymerization is not complete.
With the support of a mercaptosilane, a butadiene-containing, protective colloid-stabilized dispersion is prepared with partially hydrolyzed polyvinyl alcohol in WO 97/15603. The limitation to the mercaptosilane and the high costs of the silane constitute a disadvantage of this process.
Accordingly, there remains a need for improved processes for the preparation of protective colloid-stabilized, emulsifier-free aqueous dispersions of copolymers.
The object of the invention was to provide a process for the preparation of emulsifier-free, protective colloid-stabilized aqueous dispersions of styrene/butadiene copolymers which overcomes the described disadvantages described above, in particular the regulation of the viscosity using organic solvents, requires no protective colloids or mercaptosilanes which are expensive to prepare and makes it possible to establish the mean particle diameters below stable colloid dimensions of 1,000 nm and permits polymerization to a conversion of  greater than 99% and offers the possibility of preparing readily redispersible plastics powders by drying the dispersion.
The invention relates to a process for the preparation of protective colloid-stabilized, emulsifier-free, aqueous dispersions based on at least two monomers selected from vinylaromatics and 1,3-dienes in the presence of auxiliaries, optionally further comonomers in an amount of 0.1% by weight to 20% by weight, based on the total amount of monomers, wherein 0.01 to 0.4% by weight of an oil-soluble regulator is added to the reaction vessel at a conversion of 60 to 80%, based on 100% of the monomers, and a further 0.01 to 0.4% by weight of an oil-soluble regulator is added at a conversion of 80% to 95%, based on 100% of monomers.
Thus, in one embodiment, the present invention provides a process for the preparation of a protective colloid-stabilized, emulsifier-free, aqueous dispersion of a copolymer, comprising:
polymerizing in water at least two monomers selected from the group consisting of vinylaromatics and 1,3-dienes, at least one protective colloid, and other auxiliaries,
wherein
0.01 to 0.4% by weight of an oil-soluble regulator is added to the polymerization at a conversion of 60 to 80%, based on 100% of the monomers, and
an additional 0.01 to 0.4% by weight of an oil-soluble regulator is added to the polymerization at a conversion of 80% to 95%, based on 100% of the monomers.
The present invention also provides a process for the preparation of redispersible powders, comprising preparing the dispersion as described above, and formulating the dispersion into a redispersible powder.
The present invention also provides a process for the preparation of adhesive compositions, filling compounds, mortars, renders, gypsum building materials, and emulsion paints, comprising preparing the dispersion according to the process described above, and formulating the dispersion into an adhesive composition, filling compound, mortar, render, gypsum building material, or emulsion paint.
A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description.